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Dec. 14, 2012: Sandy Hook Elementary School

Dec. 14, 2012: A gunman at a Sandy Hook Elementary School in Newtown, Conn. killed 27 people, including 20 children. The attack appeared to be the nation's second-deadliest school shooting, exceeded only by the Virginia Tech massacre in 2007. Pictured: In this photo provided by the Newtown Bee, Connecticut State Police lead children from the Sandy Hook Elementary School following the shooting.

Credit: Shannon Hicks,AP Photo/Newtown Bee

Feb. 27, 2012: Chardon High School

Feb. 27, 2012: Three students were killed and two wounded in a shooting spree that started in a school cafeteria in Chardon, Ohio, as students waited for buses to other schools. Police charged T.J. Lane, who was 17 at the time, as an adult. Pictured: A distraught Ava Polaski, a sophomore, leaves school grounds with her mother Misty Polaski following the shooting on Feb. 27, 2012.

Feb. 14, 2008: Northern Illinois University

Feb. 14, 2008: Former student Steven Kazmierczak, 27, opened fire in a lecture hall at Northern Illinois University in DeKalb, Ill., fatally shooting five students and wounding 18 others before committing suicide. Pictured: Five crosses representing the five victims are seen at the campus of Northern Illinois University in DeKalb, Illinois, on February 15, 2008.

Credit: Amanda Rivkin/AFP/Getty Images

April 16, 2007: Virginia Tech

April 16, 2007: Seung-Hui Cho, 23, fatally shot 32 people in a dorm and a classroom at Virginia Tech in Blacksburg, then killed himself. Pictured: Virginia Tech senior Angela Lucero (L) and Mary O' Mara battle tears at a memorial service on the field of the football stadium to honor the victims, April 17, 2007 in Blacksburg, Virginia

Credit: Photo by Scott Olson/Getty Images

Oct. 2, 2006: West Nickel Mines Amish School

Oct. 2, 2006: Charles Carl Roberts IV, 32, shot to death five girls at West Nickel Mines Amish School in Pennsylvania, then killed himself. Pictured: An Amish couple talk to a fire/police officer (L), October 3, 2006 in the town of Nickel Mines, Pennsylvania a day after Roberts opened fire in the one-room Amish school there.

Credit: STAN HONDA/AFP/Getty Images

March 21, 2005: Red Lake Indian Reservation

March 21, 2005: Jeffrey Weise, 16, shot and killed five schoolmates, a teacher and an unarmed guard at a high school on the Red Lake Indian Reservation in Minnesota before taking his own life. Weise had earlier killed his grandfather and his grandfather's companion. Pictured: Robert Cook (center), a Chippewa Indian from the Red Lake Indian Reservation, prays at St. Phillip Church during a memorial service to mourn the victims killed on the reservation.

Credit: Photo by Scott Olson/Getty Images

Oct. 28, 2002: University of Arizona

Oct. 28, 2002: Robert Flores Jr., 41, who was flunking out of the University of Arizona nursing school, shot and killed three of his professors before killing himself. Pictured: University of Arizona nursing professor Barbara Monroe is shown in this undated photo from the University of Arizona in Tuscon, Arizona. Monroe was one of three professors shot and killed by Flores.

Credit: Photo by University of Arizona/Getty Images

March 5, 2001: Santana High School

March 5, 2001: Charles "Andy" Williams, 15, killed two fellow students and wounded 13 others at Santana High School in Santee, Calif. Pictured: A student at Santana High School wipes away tears while walking with classmates, after the shooting rampage at the high school.

Credit: Photo by David McNew/Newsmakers

April 20, 1999: Columbine High School

April 20, 1999: Students Eric Harris, 18, and Dylan Klebold, 17, opened fire at Columbine High School in Littleton, Colo., killing 12 classmates and a teacher and wounding 26 others before killing themselves in the school's library. Pictured: Students from Columbine High School watch as the last of their fellow students are evacuated from the school building, April 20, 1999 following the shooting spree at the school.

Credit: MARK LEFFINGWELL/AFP/Getty Images

May 21, 1998: Thurston High School

May 21, 1998: Two teenagers were killed and more than 20 people hurt when Kip Kinkel, 17, opened fire at Thurston High School in Springfield, Ore., after killing his parents. Pictured: Spingfield, Oregon mug shot of Kip Kinkel, May 22, 1998.